Lighting quality refers to how well a lighting source meets certain criteria like visual functionality, visual comfort, safety, and aesthetic appeal. Applying these quality indicators correctly can enhance the lighting experience in your space, especially now when LED lighting dominates. Using these indicators while purchasing LED light sources can help you achieve optimal results with minimal effort. Below, we’ll discuss the primary indicators of lighting quality.
1. Color Temperature
Color temperature describes the hue of white light. White light varies from reddish to bluish tones, measured in Kelvin (K). For indoor lighting, the color temperature typically ranges from 2800K to 6500K.
White light, much like sunlight, consists of different colors—red, green, and blue being the most significant. The illustration on the right shows how sunlight breaks down into multiple colors through a prism.
White light’s color is described using color temperature. More blue light components make the white light appear cooler (like midday winter sunlight in the north), whereas more red light components make it warmer (like sunrise or sunset). Color temperature is the sole way to describe white light color.
Artificial light sources also blend various colors to create white light, which we similarly describe using color temperature. To analyze white light physically, we often employ spectral analysis, which requires specialized instruments. The following figure shows several spectra.
As seen here, sunlight has a rich and complete spectral composition. Artificial light sources like incandescent bulbs and LEDs offer better spectral distributions compared to high-pressure sodium lamps and fluorescent lights. Fluorescent lamps contain harmful UV components, while incandescent bulbs consume a lot of energy and are gradually being phased out. LED light sources are becoming the dominant choice for lighting.
When describing white light color, people often categorize it as warm white, natural white, daylight white, or cool white. This broad classification doesn’t fully capture the quality of illumination. Instead, color temperature is a more precise descriptor. Typically, LED white color temperatures correspond to specific white light colors as illustrated below (a brief description). The international standard for color temperature differentiation is based on the CIE1931 chromaticity diagram, as shown in the right image:
Accurate color temperature values require professional instruments for testing. When buying lighting products, describing color temperature can be tricky. To provide a more intuitive understanding, we’ve created a visual guide to help distinguish the color temperature of lighting products:
2. Color Rendering
Color rendering measures how well a light source reduces the color of the surface of the illuminated object. It’s represented by the color rendering index Ra, which ranges from 0 to 100. The closer the Ra value is to 100, the better the color rendering, and the truer the colors of the illuminated object appear. Testing the color rendering of a light source requires professional equipment.
From the spectrum of sunlight, we see that sunlight offers the richest spectrum and is the best light source for color rendering. Artificial light sources always render colors less effectively than sunlight. To evaluate the color rendering of artificial light sources, the simplest approach is to compare the color of your palm or face under sunlight versus artificial light. The closer the colors match under sunlight, the better the color rendering. Alternatively, hold your hand up to the light source and observe its color. If your palm appears gray or yellow, the color rendering is poor. If it looks reddish, the color rendering is good (see below):
For LED light sources, Ra can be roughly divided into three levels: Ra < 69, 70 ≤ Ra < 79, and Ra ≥ 80. For high-quality indoor lighting, a light source with Ra > 80 is recommended. The color rendering index and the degree of color reduction can be compared visually in the following figure:
China adopts the CIE testing standard. For LED light sources, the general color rendering index Ra may not fully reflect the color rendering performance. CIE hasn't provided a new testing standard either. For users seeking high color rendering ability, referring to the Planck curve in the CIE1931 chromaticity diagram (the black line in the figure below), the closer the test coordinates are to the Planck curve, the better the color rendering ability.
3. Source Illuminance Value
Illuminance refers to the luminous flux emitted by the light source onto the surface of the object being illuminated per unit area. It indicates the brightness or darkness of the illuminated surface and is measured in lux (lx). Higher illuminance values mean the object is brighter.
The illuminance value is closely related to the distance between the light source and the object being illuminated. The farther the distance, the lower the illuminance value. It's also affected by the light distribution curve of the luminaire. A smaller light output angle means higher illuminance, while a larger angle results in lower illuminance. Measuring illuminance requires specialized instruments.
From a photometric perspective, luminous flux is the primary indicator. As a lighting product, it reflects the brightness or darkness of the illuminated surface. Illuminance value provides a more accurate description of the lighting effect. The indoor illuminance level impacts how bright or dim the space feels. Too much or too little illumination can affect eye health. Thus, indoor illuminance should align with national standards (Table 1). Recommended values shouldn't exceed or fall below this range excessively.
Table 1: Residential Building Lighting Illuminance Standards
4. Luminaire Light Distribution Curve
The indoor lighting effect depends on the luminaire layout and its light distribution curve. Good lighting results come from proper layouts and effective light distribution applications. The arrangement and light distribution of the luminaire determine the visual function and comfort of indoor lighting, creating depth and layers in the lighting space. Proper light distribution enhances the overall lighting quality.
The purpose of a luminaire is to fix and protect the light source, decorate the environment, and redistribute the light output of the source so that it emits light at the designed angle. This redistribution is called the light distribution of the luminaire. The light distribution curve describes the luminaire’s light output pattern. Smaller angles produce a brighter sensation. The light distribution curve is tested using specialized instruments. The figure below shows common light distribution curves for luminaires:
The light distribution depends on the luminaire's lighting function. Track spotlights and ceiling lights typically use 15° or 30° light distribution, downlights use 30° or 60°, bulbs and ceiling lights generally use 120°, and panel lights use 60° to 90°.
5. Luminous Efficiency of the Light Source
The brightness of the light source is described by luminous flux, measured in lumens (lm). Higher luminous flux indicates a brighter light source. The ratio of the luminous flux to the power consumption of the light source is called the luminous efficiency, measured in lm/W (lumens per watt).
Luminous efficiency is a crucial indicator for assessing the quality of a light source. Higher luminous efficiency means the light source is more energy-efficient. LED light sources have a luminous efficiency of around 90–130 lm/W, energy-saving lamps range from 48–80 lm/W, incandescent lamps have 9–12 lm/W, and poor-quality LED light sources only reach 60–80 lm/W. High luminous efficiency and good light source quality go hand in hand.
6. Luminaire Efficiency
Indoor lighting rarely uses standalone light sources. Instead, they’re placed inside luminaires. Once placed, the luminaire’s light output is usually lower than that of the individual light source. The ratio of these outputs is called luminaire efficiency. Higher efficiency indicates better luminaire manufacturing quality and energy-saving performance. Luminaire efficiency is an essential metric for evaluating luminaire quality.
The relationship between the luminous efficiency of the light source, the luminaire efficiency, and the illuminance value is that the luminaire’s luminous flux output is proportional to its efficiency. The illuminance value is proportional to the luminous efficiency of the light source, and it’s also related to the luminaire’s light distribution curve. High illuminance doesn’t necessarily mean high luminous flux. Even if the light source has high luminous efficiency, placing it in a low-efficiency luminaire significantly reduces the luminaire’s output. Higher luminous efficiency of the light source and luminaire efficiency indicate better energy-saving performance. These two metrics are key indicators for assessing high-quality lighting products.
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